1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device operating in wavelength bands from the visible to ultraviolet regions, such as a light emitting diode (LED) or a laser diode (LD) and a method for manufacturing the same.
2. Description of the Related Art
In recent years, nitride semiconductors, typically gallium nitride conductors, have come to be favored for use as materials for light emitting devices in the green to ultraviolet regions.
Whereas a light emitting device using a nitride semiconductor usually has, in addition to a light emitting layer (commonly known as an active layer) having a multi-quantum well structure, p-type and n-type nitride semiconductor layers for current injection above and underneath the light emitting layer, the resistance of the p-type nitride semiconductor layer itself is known to be extremely high because the p-type layer is significantly lower than the n-type layer in carrier activation rate, only a few percent.
Whereas AlGaN containing aluminum (Al) is usually used for the clad layer for enclosing light, as the carrier activation rate falls in a reverse relation to a rise in Al content especially in a p-type layer, it is generally known that a good ohmic property can hardly be obtained between an Al-containing p-type nitride semiconductor layer and a metallic material.
For this reason, in a p-type contact layer in which an ohmic electrode is to be formed, gallium nitride (GaN) not containing Al and doped with p-type impurities in a high concentration (see for instance JP-A-6-268259) is used, or cases in which a good ohmic property was obtained by using InGaN, which is narrower in forbidden band width and higher in carrier activation rate than GaN, are disclosed (see for instance JP-A-2003-7998).
Cases in which a good ohmic property is obtained and the self-absorption of light is restrained by using an AlGaN layer for the p-type contact layer a double-layered AlGaN structure differing in Al content and in p-type impurities for some light emitting wavelength bands to restrain self-absorption of light are also disclosed (see for instance JP-A-2001-203385).
Another case in which, in order to an oxide-based electrode, a sufficient ohmic property is obtained between the electrode and the p-type contact layer by using an AlGaN layer lower in Al content than the clad layer as the contact layer is disclosed JP-A-2000-36616).